Deprecate basic_search parameter

What does this MR do and why?

Deprecate basic_search parameter and add documentation for search_type. search_type was added in Fix how search type is determined (!161999 - merged) as can be one of basic, advanced or zoekt which replaces the need for the basic_search param.

MR acceptance checklist

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How to set up and validate locally

1. Perform a normal advanced search
2. Add search_type=basic to the URL and see that basic search was used
3. Add basic_search=true to the URL and see that advanced search was used - the basic_search param doesn't have an impact.

Related to #477333 (closed)